VSEPR Theory

• Valence Shell Electron Pair Repulsion (VSEPR) — electron pairs (bonding + lone) arrange to minimise repulsion.
• To predict electron geometry and molecular shape for AB_n and lone-pair cases.
• Bond angle trends when lone pairs replace bonding pairs (e.g., H₂O < tetrahedral angle).
• Link to hybridisation intro (sp, sp², sp³) for NCERT Class 11 bonding chapter.

Level 1 — Electron pair counting and basic shapes

Verbal: Count electron domains on central atom = bonds (single/double/triple each count as 1 domain) + lone pairs. Domains spread out in 3D.

Symbolic: Steric number = bonds + lone pairs on central atom; electron domains dictate geometry; tan θ = v²/(rg) for ideal banking (physics link).

Steric number table:

Level 2 — Lone pairs and molecular shape

Lone pair–bond pair repulsion > bond–bond repulsion → lone pairs compress angles.

Hybridisation link (same steric number):

• 2 domains → sp linear
• 3 → sp² trigonal planar
• 4 → sp³ tetrahedral

Dipole and shape: Symmetric shapes (CH₄, CO₂) may have μ = 0 even with polar bonds.

NCERT spotlight — Hybridisation connection

Steric number 4 with no lone pairs gives tetrahedral CH4 and sp3 hybridisation. Steric number 3 with no lone pairs gives trigonal planar BF3 and sp2. Steric number 2 gives linear BeCl2 and sp.

Lone pair effects: SF4 has seesaw shape; ClF3 is T-shaped; XeF2 is linear with three lone pairs on Xe — know common JEE/NEET examples.

Dipole moment: Symmetric molecules like CCl4 have zero net dipole despite polar bonds — vector cancellation depends on geometry from VSEPR.

Predict shapes and bond angles of CH₄, NH₃, and H₂O using VSEPR. All have 4 electron domains on central atom.

Step 1 — CH₄: 4 bonding, 0 lone → tetrahedral, H−C−H ≈ 109.5°.
Step 2 — NH₃: 3 bonding, 1 lone → pyramidal; lone pushes H−N−H to ~107°.
Step 3 — H₂O: 2 bonding, 2 lone → bent; H−O−H ~104.5° (two lone pairs compress more).
Step 4 — Same steric number 4 → sp³ hybridisation on C, N, O (different geometry due to lone pairs).
Step 5 — μ: CH₄ = 0; NH₃ ≠ 0; H₂O ≠ 0.

Applications — molecular polarity and reactivity

CO2 linear nonpolar — greenhouse gas but no dipole; H2O bent polar — solvent for ionic species. PCl5 trigonal bipyramidal: axial vs equatorial bond lengths differ slightly due to repulsion — explains substitution patterns in phosphorus chemistry preview. SF6 octahedral, symmetric, nonpolar — used as electrical insulator in high-voltage equipment.

Deep dive — stereochemistry and lone pair examples

XeF2 linear: Xe has 5 electron domains (2 bonds + 3 lone pairs) — arrange lone pairs equatorial in trigonal bipyramidal electron geometry → linear molecular. ClF3 T-shaped: 5 domains, 2 lone equatorial. SF4 seesaw: 5 domains, 1 lone equatorial. BrF5 square pyramidal: 6 domains, 1 lone. IF7 pentagonal bipyramidal no lone. Bond angle depression: CH4 109.5°, NH3 107°, H2O 104.5° — lone pair repulsion sequence explains trend quantitatively qualitatively for exams. Hybridisation-sp3d sp3d2 for expanded octet central atoms — links VSEPR count to orbital mixing. Dipole vector addition: tetrahedral CCl4 cancel; trigonal pyramidal NF3 net dipole; see-saw SF4 polar. Isomers structural vs spatial — VSEPR predicts geometry enabling cis-trans isomerism preview in alkenes Class 11 organic introduction if syllabus overlaps.

Review and practice drill

Review checklist: (1) Count electron domains on central atom. (2) Lone pairs change molecular shape. (3) Hybridisation linked to steric number. (4) Dipole from geometry. Practice: NH3 pyramidal, bond angle about 107 degrees.

• What is the shape of BF₃? Of ClF₃ (3 bonds + 2 lone on Cl)?
• Why is H₂O bent but CO₂ linear?
• Steric number of S in SF₆?

Open the Practice tab for graded questions on VSEPR.